1. Field of the Invention
The present invention relates to a standard sample used for instrumental measurement of Saybolt color with a photoelectric colorimeter (color and color difference meter) which is a color testing method for petroleum products such as kerosene and liquid paraffin, and a method of instrumental measurement of Saybolt color of a petroleum product using said sample.
2. Prior Art
There are two color testing methods for petroleum products, that is, the testing method for Saybolt color and the testing method for ASTM color which are prescribed in JIS K 2580.
Of these, the testing method for Saybolt color is applied to petroleum products having a high transparency, such as kerosene, liquid paraffin and paraffin wax, and symbolizes the brightness of a sample generally obtained by comparing the transmitted light through a standard colored glass with that through the sample by the use of a Saybolt chromometer. The color scale ranges from +30, the brightest, to -16, the darkest.
The Saybolt color as prescribed in JIS K 2580 is symbolized by the brightness of a sample obtained by comparing the transmitted light through a standard colored glass with that through the sample under test, and the standard colored glass is prescribed in its dispersion on the basis of the XYZ colorimetric method and the spectral transmittance by the use of the CIE standard illuminants C in accordance with JIS Z 8722.
However, since the judgement of the Saybolt color by the use of the Saybolt chromometer is visually conducted by a person in charge of measurement, the current situation is that a personal difference is apt to arise, thus bothering the person in the judgement. When the hue of a liquid is similar to that of a standard colored glass, the measured values agree with each other owing to small personal difference in brightness. However, when the hues of the above differ from each other, the personal difference increases.
Under such circumstances, automatization of such a testing method without resort to the visual measurement is eagerly desired at the present time.
In view of the above, an attempt was made by the present inventors to investigate whether or not a commercially available photoelectric colorimeter can be used for measuring the Saybolt color.
A measuring instrument used must be one in which a favorable correlation with the testing method as prescribed in JIS K 2580 is obtained in the entire range of the Saybolt color ranging from +30 to -16. Furthermore, a universal method of testing the color of material, especially that of liquid is desired.
As a method of measuring the color of material, the XYZ color system recommended by Commission Internationale de 1'Eclairage (hereinafter abbreviated to "CIE") and prescribed in JIS Z 8722, and, as a method of specifying the color, the L*a*b* color system also recommended by CIE and prescribed in JIS Z 8729 are universal methods.
As a result of further investigation on the above-mentioned matter, it was decided by the present inventors to increase the optical path length of a commercially available measuring instrument (photoelectric colorimeter) from 20 mm to 100 mm to enhance the accuracy measurement. As a result, it was found that a favorable correlation existed between the Saybolt color (S: provisional name) and a color difference .DELTA.E*ab (E: provisional name) based on the L*a*b* color-specification method, as represented by the following calibration formula: EQU S=(.alpha./logE-.beta.)+.gamma.
wherein .alpha., .beta. and .gamma. are each a constant.
As the above E value varies with an instrumental error depending on an instrument, it cannot be kept constant even for the same sample. It was necessary, therefore, to prepare a standard sample for Saybolt color to calibrate each instrument (photoelectric colorimeter) using the above sample.
Such a standard sample is required to be composed of a base material similar to a petroleum product, and have low volatility, low deterioration such as oxidation, with the elapse of time and good color stability.
There are available a method using a Hazen colorimetric standard solution prescribed in JIS K 2421 as the standard color solution, a method using platinum-cobalt scale prescribed in ASTM D-1209 and D-365 and the like. However, these methods are not suitable for measuring the Saybolt color, because a dark Saybolt color causes a different hue and each of the above solutions comprises an aqueous acid solution which corrodes a precision instrument to be used, thus making the operation troublesome.